Wiring board and soldering method therefor

ABSTRACT

A circuit board for surface mounting by solder flow an electronic component having narrow lead pitches, the board having a first solder leading land having a first side and located on the circuit board adjacent to a land for mounting an electronic component, and a second solder leading land located next to the first solder leading land and opposite the first side of the first solder leading land. A method for flow soldering a surface mounted electronic component on such a circuit board, wherein the second solder leading land is positioned at a rear end of the circuit board against a direction of a movement of the circuit board when the circuit board moves toward a solder flow.

TECHNICAL FIELD

The present invention relates to a circuit board on which surfaceelectronic components are mounted by soldering at narrow lead pitches asis the case with quad flat package integrated circuits (hereinafter,QFPIC). More specifically, the present invention relates to a circuitboard preferable for mounting electronic components at narrow leadpitches by a flow soldering method.

BACKBROUND ART

As an example of narrow-pitch mounting of electronic components, aconventional flow soldering method for QFPIC is described with referenceto FIG. 6. A QFPIC 2, a surface-mount component, is approximately squareshaped with a plurality of leads 3 aligned along its four sides. Lands 4are disposed on a circuit board 1 in such a manner that they correspondwith the leads 3. Since the leads 3 are disposed at fine pitches, solderbridges often occur between leads during flow soldering. The solderbridges occur more easily when pins are disposed successively at rightangles to a direction in which the circuit board moves during the flowsoldering. To avoid the solder bridges, as illustrated in FIG. 6, QFPICis soldered at an angle of 45 degrees against the direction the circuitboard moves during flow soldering. This construction allows a flow ofsolder to come in contact with leads and lands sequentially from a frontto a rear side, and consequently promotes the flow.

Another method of preventing the solder bridge uses a land called solderleading land which is formed at a rear portion of each side of QFPIC 2against the direction in which the circuit board moves for thesoldering. In this method, mid solder leading lands 12 and a rear sidesolder leading land 13 are formed respectively near QFPIC 2 in betweengroups of leads and lands 10 and 11 lining up respectively along the twosides in the front side and two sides in the rear side, and at the rearend of the groups of leads and lands 11 located along the two rear sidesnear QFPIC 2.

FIG. 7A is a schematic diagram showing a side view of the circuit board1. As shown in FIG. 7A, by leading solder 21 which has flowed out from aflow soldering nozzle 20, over leads and lands sequentially to thesolder leading land 13 by using surface tension, excessive accumulationof solder in the section of leads and lands is prevented from occurring.This principle works for both mid solder leading lands 12 and rear sidesolder leading land 13.

However, mounting electronic components by flow soldering at narrow leadpitches as is the case with QFPIC still entails challenges regarding howto lead an excessive solder built up around leads 3 a and lands 4 alocated at the rear ends of each side, to the solder leading land 13.The amount of solder for two lines of leads in the rear side accumulatesaround the rear ends of the sides of QFPIC against the direction inwhich the circuit board moves during flow soldering. This promotesformation of solder bridges between leads as well as lands, and thusrequires expertise regarding selection of optimum shape of lands for therear solder leading land. In FIG. 7B, an example of appropriatesoldering is indicated by numeral 22. Around a final lead 3 a however,excessive solder may cause solder bridge 23 between leads.

In the case of lead-free solder which has been recently attractingattention as an environmentally-friendly solder, the above describedproblem is experienced more often due to the reasons described below.

Since lead-free solder does not contain lead which carries the importantrole as an agent to help lower the melting point and improve fluidityand wettability, its fluidity and wettability is lower, thus solderbridges are formed more frequently.

In a process of mass production of the circuit boards, when a solderbridge is formed during the process of the flow soldering, the problemneeds to be repaired manually in a following process. As a result,required man-hours are increased, and yields lowered. Furthermore,excessive heat stress associated with manual soldering poses apossibility of damaging components.

The present invention provides a circuit board and its manufacturingmethod which address the conventional problems described above andachieves smooth flow soldering for QFPIC.

DISCLOSURE OF THE INVENTION

The circuit board of the present invention comprises a first solderleading land disposed at a portion on a circuit board corresponding to acorner of a QFPIC which is to be mounted, and a second solder leadingland disposed approximately opposite to the QFPIC side of the firstsolder leading land.

While the circuit board is soldered by a flow soldering method, thecircuit board is moved toward a flow solder tank in such a manner thatthe corner where the first and the second solder leading lands aredisposed is located at a tail.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a circuit board of a firstpreferred embodiment of the present invention.

FIG. 2 is a schematic diagram showing a circuit board of a secondpreferred embodiment of the present invention.

FIG. 3A is a schematic diagram showing a circuit board of a thirdpreferred embodiment of the present invention.

FIG. 3B is a schematic diagram showing solder leading lands and a solderguiding land.

FIG. 4 is a schematic diagram showing a circuit board of the thirdpreferred embodiment of the present invention.

FIG. 5 is a schematic diagram showing a circuit board of a fourthpreferred embodiment of the present invention.

FIG. 6 is a schematic diagram showing a circuit board of a prior art.

FIG. 7A is a schematic diagram showing a QFP solder leading land.

FIG. 7B is a schematic diagram showing a function of the QFP solderleading land.

BEST MODE FOR CARRYING OUT THE INVENTION

The circuit board of the present invention has at least a QFPIC mountedthereon, and comprises a first solder leading land disposed at a portionon the circuit board corresponding to at least one of the corners ofQFPIC, and a second solder leading land disposed next to and at anopposite to QFPIC side of the fist solder leading land.

In this construction, excessive solder accumulated at around rear endsof the leads and lands of QFPIC can be guided by surface tension to thefirst soldering land, and further more, to the second solder leadingland in a case that an amount of the solder exceeds the capacity of thefirst solder leading land. As such, formation of a solder bridge can beprevented around leads and lands of QFPIC.

In another embodiment of the present invention, the first solder leadinglands are disposed at portions on the circuit board corresponding toopposing two corners of QFPIC and the second solder leading lands nextto and against the first solder leading lands on the opposite side ofQFPIC.

With this construction, the circuit board can be moved with eithercomer, i.e. the comer where the first solder land and the second solderland is disposed, being as a front during the flow soldering process.This construction enhances degrees of freedom for soldering. Forinstance, when a plurality of pieces of circuit boards are formed on oneboard with one piece of circuit board 180 degrees rotated to next piece,the direction in which the QFP moves becomes opposite to one anotheraccordingly, however, no special consideration is required for patternswhen determining to which direction the circuit board should move at atime of flow soldering. This construction further facilitates patterndesigning for the circuit board since QFPIC can be mounted withoutrestriction of direction.

In still another embodiment of the present invention, the first solderleading land is divided into two pieces and the two pieces are disposedwith a diagonal line to a corner of QFPIC, where the first solderleading lands are disposed, in between. And the two pieces are inalignment with the QFP leads disposed along the two sides which form thecorner of QFPIC.

With this construction, excessive solder, which accumulates along thetwo sides forming the corner of QFPIC where the first and second solderleading lands are disposed, is absorbed by the first solder leadinglands disposed adjacent to the final leads located at the rearmost ofthe lines of the groups of the leads; i.e. leads which are located atthe rearmost when the circuit board moves toward the flow solder tankduring the flow soldering process. Furthermore, if solder overflows fromthe first solder leading lands, it is then guided to the second solderleading land. Consequently, problems such as solder bridges formingaround the groups of leads, especially in the area of the final leads,can be prevented effectively.

Moreover, even when solder bridges are formed simultaneously in the tworear sides of QFPIC between the final leads and the adjacent firstsolder leading lands, since the first solder leading lands are dividedinto two and allocated for the two different sides, short circuit of theelectric connection can be avoided and thus connection stability furtherimproves.

In yet another embodiment of the present invention, the first and secondsolder leading lands of the same shape against the center of QFPIC aredisposed at each of the four corners of QFPIC.

It is easy to form the first and second solder leading lands of the sameshape at the four corners of QFPIC. This construction enhances thedegree of freedom regarding the direction in which the circuit boardmoves during flow soldering.

In the foregoing constructions, the second solder leading land can bethe same as or larger than the first solder leading land.

Making the capacity of the second solder leading land larger than thatof the first solder leading land facilitates flow of solder to thesecond solder leading land thereby enhancing effectiveness of solderbridge prevention.

In addition, at least one of the first and second solder leading landscan be provided with a through-hole formed at a section of the land ofcopper foil.

The through-hole helps release gas generated during soldering from thesolder leading lands efficiently and thus suppresses incompletesoldering caused by gas bubbling. Consequently, quality of soldering canbe further improved.

It is also possible to form the first solder leading lands, which aredisposed in the first corner of QFPIC in alignment with the groups ofleads lining up along the two sides of QFPIC, such that they taper downtowards the direction of the first corner from either second or thirdcorner, which are not diagonally opposite the first corner of QFPIC.

With this construction, excessive solder from the first solder leadingland can be easily guided to the second solder leading land. Shouldexcessive solder accumulate, it could be led to the second solderleading land effectively. Consequently, the possibility of back flow ofexcessive solder from the first solder leading land to the final leadsor lands on the side of QFPIC is lowered, thus yields and reliability ofthe circuit board can be improved.

At least the second solder leading land is formed such that it tapersdown toward the opposite direction to QFPIC against the first cornerwhere the second solder leading land is disposed.

This shape helps cut out unnecessary solder which has been guided to thesecond solder leading land while the circuit board moves. Thus, thepossibility that excessive solder remains on the circuit board andcauses short circuit with other lands can be lowered significantly.

As for the lead-free solder, its inferior properties such as low influidity and poor wettability are cancelled out by the soldering methodput forth by the present invention, and occurrence of solder bridgesdecreases. Using lead-free solder at the same time achievesenvironmentally friendly products.

The preferred embodiments of the present invention are described belowwith reference to the accompanying drawings. In the description thecomponents which are of the same construction as that of the example ofthe prior art are labeled with the same numerals.

The First Preferred Embodiment

FIG. 1 shows the printed circuit board of the first preferred embodimentof the present invention. In the case of the solder leading land of theprior art as shown in FIG. 6, the whole amount of excessive solder needsto be accepted in one solder leading land without exception. In order topromote the flow of excessive solder to the solder leading land bysurface tension of the solder itself, the area of the land and the levelof the surface tension need to be optimized. Due to these problems, theamount of the solder guided to the solder leading land varies,consequently, solder bridges cannot be fully prevented.

In the present embodiment, the land is divided into first and secondsolder leading lands 13 a and 13 b, that are formed by a copper foil 14and a solder resist 15 covering the copper foil 14, in order to guideexcessive solder stepwise. With this construction, excessive solder isguided to the first solder leading land 13 a at the rearmost ends of thesides of QFPIC against the direction of the flow soldering.Successively, the excessive solder is guided to the second solderleading land 13 b. Thus the amount of solder remaining in the firstsolder leading land is reduced. Consequently, adhesion of solder flowedback from the solder leading land 13 a to adjacent leads and lands ofQFPIC and an occurrence of a formation of the solder bridges can beprevented.

In FIG. 1, the solder leading lands 13 a and 13 b are triangular,however, providing that they are disposed sequentially according to thedirection in which the circuit board moves, they do not have to betriangular; they can be circular or rectangular. However, as shown inthe same figure, at least the second solder leading land 13 b ispreferably triangular or has a shape tapering off toward the directionopposite to the direction in which the circuit board moves during flowsoldering.

The Second Preferred Embodiment

FIG. 2 shows a circuit board of the second preferred embodiment of thepresent invention. In the present embodiment, a triangular second solderleading land 13 b is disposed against a triangular first solder leadingland 13 a. The second solder leading land 13 b is larger than the firstsolder leading land 13 a. This construction facilitates a flow of soldertoward the second solder leading land 13 b, and effectively preventsadhesion of solder caused by the solder flowing back from the secondsolder leading land 13 a. In the present preferred embodiment, also, thelands do not have to be triangular.

Disposing the first and second solder leading lands 13 a and 13 b onopposing two corners against the center of QFPIC 2 as shown in FIG. 2achieves soldering of high quality even when a plurality of circuitboards being formed on one board at opposite directions to one anotherare soldered simultaneously in a flow soldering line.

The Third Preferred Embodiment

FIG. 3A shows a circuit board of the third preferred embodiment of thepresent invention.

With reference to FIG. 3B, a solder leading land and a solder guidingland are described below.

As shown in FIG. 3B, a solder leading land 20 is disposed in alignmentwith the central axis of lines of pins, which should be protected fromsolder bridges, on the rear side of the circuit board against thedirection in which it moves. The leading land tapers down as thedistance from the land needing to be protected from solder bridgesbecomes larger. The purpose of this shape is to prevent adverse currentof solder to the land needing to be protected by removing flow soldergradually from the land.

A solder guiding land 31 guides solder to the land which needs to besoldered. The solder guiding land 31 is disposed where the flow ofsolder is slow or wettability is low in order to improve soldering.Because the function of the solder guiding land 31 contrasts with thesolder leading land 20, it is effective to form the solder guiding land31 such that its area expands toward the land which needs to be solderedas shown in FIG. 3B.

In accordance with the third preferred embodiment of the presentinvention, in order to ensure the effect of the solder leading land, thetriangular first solder leading lands 13 a which taper off toward therear side against the direction of the soldering, or in other words,taper off as the distance from the land becomes larger, are disposedrespectively on the axes of the leads lining up along the two rearsides. Behind these two solder leading lands 13 a disposed one each onthe two rear sides is the second solder leading land 13 b which combinesthe two solder leading lands 13 a and leads solder toward a rear side.The second solder leading land 13 b is also triangular with areatapering off as the distance from the first solder leading lands 13 abecomes larger. As such, these three triangles form a solder leadingland. The land comprising three triangles can be produced byresist-printing on a piece of rectangular copper foil and dividing itinto three. However, in the present invention, the land is produced bycutting the copper foil into required forms as shown in FIG. 3A.

According to the above structure, as the shapes of the solder leadinglands are optimized in a manner described above, flow of excessivesolder is led more effectively and a short circuit between the finalleads can be prevented even when the two rear sides simultaneouslysuffer solder bridges between the final leads and the neighboring firstsolder leading lands 13 a. As electrical connection is secured, higherstability in soldering and improved yields can be possible.

While the shape of the solder leading lands is not limited, it ispreferable to make it according to the shapes shown in FIGS. 3A and 3B.

As FIG. 4 shows, if the solder leading lands described above aredisposed in all of the four corners, the circuit board can be placedfreely without limitation regarding the direction in which the circuitboard moves for flow soldering. As such, even when a plurality ofcircuit boards are formed on one board with 180 degrees rotation to oneanother and direction in which each circuit board moves becomes oppositeaccordingly, soldering can be conducted without any limitation. Duringthe designing process of the circuit board, pattern designing can beexpedited since direction of QFPIC does not have to be put intoconsideration.

In this preferred embodiment, basic shape of the lands is triangle.While this shape is easily patterned and looks beautiful, the shape isnot limited to triangle.

The Fourth Preferred Embodiment

FIG. 5 shows a circuit board of the fourth preferred embodiment of thepresent invention. In the present embodiment, the second solder leadingland 13 b disposed at the rear end according to the third preferredembodiment is provided with a through-hole 16 which penetrates throughthe circuit board. The through-hole may also be provided to the firstsolder leading land 13 a.

The through-hole helps release gas which is generated during the processof soldering, thereby preventing incomplete soldering which couldotherwise be caused by gas bubbling and achieving soldering withsuperior stability. As has been the case with the previous embodiments,the shape of the lands is not limited to particular shape.

In the previous embodiments thus far described, QFPIC is taken up as anexample of the application of the present invention, however, thepresent invention is also applicable to circuit boards on whichcomponents are mounted at narrow lead pitch.

INDUSTRIAL APPLICABILITY

According to the circuit board of the present invention with the firstand second solder leading lands, excessive solder guided to the secondsolder leading land can be cut off smoothly against the movement of thecircuit board. Consequently, the possibility that excessive solderremains on the circuit board and triggers a short circuit with anotherland can be significantly reduced. In this manner, stable soldering ofQFPIC can be achieved even in a flow soldering process.

Application of the present invention to soldering using lead-free solderoffsets inferior properties of the lead-free solder such as low fluidityand poor wettability, and thus further reduces occurrence of solderbridges. Furthermore, environmentally-friendly products can beconstructed.

1. A circuit board for surface mounting an electronic component withnarrow lead pitches using solder, said circuit board having a rear enddesignated in accordance with a direction in which the board is intendedto move, the circuit board comprising: a first solder leading land and asecond solder leading land located on the circuit board near a cornerlocated at said rear end of the circuit board, and for facilitatingmounting of an electronic component thereon, wherein the first solderleading land is located on the circuit board adjacent to a land formounting an electronic component, the first solder leading land having arear end, said solder leading lands for locating such an electroniccomponent therebetween, and the second solder leading land is locatedadjacent the rear end of the first solder leading land.
 2. The circuitboard of claim 1, wherein the electronic component is a quad flatpackage integrated circuits (“QFPIC”).
 3. The circuit board of claim 2,wherein said first and second solder leading lands are located on thecircuit board corresponding to at least two opposite corners of theQFPIC, the two opposite corners forming a diagonal line.
 4. The circuitboard of claim 2, wherein said first solder leading land is divided intotwo portions corresponding to two sides of the QFPIC and respectivelylocated in alignment with each of the two sides.
 5. The circuit board ofclaim 1, wherein an area of the second solder leading land is at leastequal to an area of the first solder leading land.
 6. The circuit boardof claim 1, wherein at least one of the first and the second solderleading lands has a through-hole which penetrates the substrate.
 7. Thecircuit board of claim 1, wherein at least the first solder leading landtapers off toward a direction opposite the electronic component.
 8. Thecircuit board of claim 1, wherein at least the second solder leadingland tapers off toward a direction opposite to said first solder leadingland.
 9. The circuit board of claim 1, wherein the solder is lead-free.10. A method for flow soldering a surface mounted electronic componenthaving narrow lead pitches to a circuit board, said circuit board havinga rear end designated in accordance with a direction in which the boardis intended to move, the board comprising a first solder leading landand a second solder leading land located on the circuit board near acorner located at said rear end of the circuit board, and forfacilitating mounting of an electronic component therebetween, whereinthe first solder leading land is located on the circuit board adjacentto a land for mounting an electronic component, the first solder leadingland having a rear end, said solder leading lands for locating suchelectronic component therebetween, and the second solder leading land islocated adjacent the rear end of the first solder leading land, themethod comprising: positioning the second solder leading land at therear end of the circuit board against a direction of a movement of thecircuit board when the circuit board moves toward a solder flow.